STAINING, PRACTICAL AND THEORETICAL 



course of work carried out between the years 1947 and 1957 I 

 had occasion to formulate and prepare a large number of tri- 

 chrome mixtures of acid dyes for demonstrating keratins, etc., 

 and had observed that under suitable conditions, acid dyes 

 having molecular weights between 350 and 590 appeared to give 

 satisfactory results with keratins. The lower molecular weights 

 between these two limits gave excellent results. It was stated at 

 the time that these were only general observations, and exceptions 

 would no doubt be found. Exceptions, in fact, have since been 

 found (MacConaill & Gurr, 1959; Gurr & MacConaill, 1959). 

 For instance, patent blue A (molecular weight 1405) did not 

 behave as was expected. The reason for this might be put down 

 to the fact that the molecule of this dye is made up of two uni- 

 valent dye-ions attached to one divalent cation. In solution 

 ionization takes place with the release of these two dye-ions 

 which are capable of association separately with two univalent 

 cations to form two dye-molecules of approximately half the 

 molecular weight of the original dye. A few other exceptions 

 were also found. Although it would appear that other factors 

 besides molecular weight — e.g. molecular volume, molecular 

 dimensions, side-chains, diffusion phenomena — influence the 

 penetrative powers of dyes, a study of literature over the past 

 eighty years or so does appear to show that most acid dyes, when 

 in competition with each other generally behave in accordance 

 with the original hypothesis. 



In many cases the molecular weight of an acid dye does give a 

 reasonably clear indication whether the dye under consideration 

 is likely to be a suitable alternative for another acid dye of around 

 the same molecular weight. A few dyes may not behave in 

 accordance with the original hypothesis, but this method of 

 selection has been found very considerably quicker, and more 

 economical in materials than the process of trying out a large 

 number of acid dyes irrespective of their molecular weights. It 

 appears that the molecular weight in the case of many acid 

 dyes has far more influence on the type of tissue elements 

 stained than has the chromophoric configurations of the dyes. 

 In fact the side chains (e.g. sulphonic, carboxylic, hydroxyl, 

 amino, imino groups, etc.) appear to have more influence on the 

 staining effects than do the chromophores of most acid dyes. 



All dyes possess a configuration known as the chromogen. 



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